Am 19.05.2021 um 15:24 hat Peter Lieven geschrieben: > Am 20.04.21 um 18:52 schrieb Vladimir Sementsov-Ogievskiy: > > 20.04.2021 18:04, Kevin Wolf wrote: > >> Am 20.04.2021 um 16:31 hat Vladimir Sementsov-Ogievskiy geschrieben: > >>> 15.04.2021 18:22, Kevin Wolf wrote: > >>>> In order to avoid RMW cycles, is_allocated_sectors() treats zeroed areas > >>>> like non-zero data if the end of the checked area isn't aligned. This > >>>> can improve the efficiency of the conversion and was introduced in > >>>> commit 8dcd3c9b91a. > >>>> > >>>> However, it comes with a correctness problem: qemu-img convert is > >>>> supposed to sparsify areas that contain only zeros, which it doesn't do > >>>> any more. It turns out that this even happens when not only the > >>>> unaligned area is zeroed, but also the blocks before and after it. In > >>>> the bug report, conversion of a fragmented 10G image containing only > >>>> zeros resulted in an image consuming 2.82 GiB even though the expected > >>>> size is only 4 KiB. > >>>> > >>>> As a tradeoff between both, let's ignore zeroed sectors only after > >>>> non-zero data to fix the alignment, but if we're only looking at zeros, > >>>> keep them as such, even if it may mean additional RMW cycles. > >>>> > >>> > >>> Hmm.. If I understand correctly, we are going to do unaligned > >>> write-zero. And that helps. > >> > >> This can happen (mostly raw images on block devices, I think?), but > >> usually it just means skipping the write because we know that the target > >> image is already zeroed. > >> > >> What it does mean is that if the next part is data, we'll have an > >> unaligned data write. > >> > >>> Doesn't that mean that alignment is wrongly detected? > >> > >> The problem is that you can have bdrv_block_status_above() return the > >> same allocation status multiple times in a row, but *pnum can be > >> unaligned for the conversion. > >> > >> We only look at a single range returned by it when detecting the > >> alignment, so it could be that we have zero buffers for both 0-11 and > >> 12-16 and detect two misaligned ranges, when both together are a > >> perfectly aligned zeroed range. > >> > >> In theory we could try to do some lookahead and merge ranges where > >> possible, which should give us the perfect result, but it would make the > >> code considerably more complicated. (Whether we want to merge them > >> doesn't only depend on the block status, but possibly also on the > >> content of a DATA range.) > >> > >> Kevin > >> > > > > Oh, I understand now the problem, thanks for explanation. > > > > Hmm, yes that means, that if the whole buf is zero, is_allocated_sectors > > must not align it down, to be possibly "merged" with next chunk if it is > > zero too. > > > > But it's still good to align zeroes down, if data starts somewhere inside > > the buf, isn't it? > > > > what about something like this: > > > > diff --git a/qemu-img.c b/qemu-img.c > > index babb5573ab..d1704584a0 100644 > > --- a/qemu-img.c > > +++ b/qemu-img.c > > @@ -1167,19 +1167,39 @@ static int is_allocated_sectors(const uint8_t *buf, > > int n, int *pnum, > > } > > } > > > > + if (i == n) { > > + /* > > + * The whole buf is the same. > > + * > > + * if it's data, just return it. It's the old behavior. > > + * > > + * if it's zero, just return too. It will work good if target is > > alredy > > + * zeroed. And if next chunk is zero too we'll have no RMW and no > > reason > > + * to write data. > > + */ > > + *pnum = i; > > + return !is_zero; > > + } > > + > > tail = (sector_num + i) & (alignment - 1); > > if (tail) { > > if (is_zero && i <= tail) { > > - /* treat unallocated areas which only consist > > - * of a small tail as allocated. */ > > + /* > > + * For sure next sector after i is data, and it will rewrite > > this > > + * tail anyway due to RMW. So, let's just write data now. > > + */ > > is_zero = false; > > } > > if (!is_zero) { > > - /* align up end offset of allocated areas. */ > > + /* If possible, align up end offset of allocated areas. */ > > i += alignment - tail; > > i = MIN(i, n); > > } else { > > - /* align down end offset of zero areas. */ > > + /* > > + * For sure next sector after i is data, and it will rewrite > > this > > + * tail anyway due to RMW. Better is avoid RMW and write > > zeroes up > > + * to aligned bound. > > + */ > > i -= tail; > > } > > } > > I think we forgot to follow up on this. Has anyone tested this > suggestion? > > Otherwise, I would try to rerun the tests I did with the my old and > Kevins suggestion.
I noticed earlier this week that these patches are still in my development branch, but didn't actually pick it up again yet. So feel free to try it out. Kevin